The replacement or repair of damaged or diseased tissues by implantation is a continuing goal of the medical profession. However, there are a number of difficulties that restrict the use of implanted materials.
In particular, implantation of interbody devices into the intervertebral disc, which may be degraded or damaged by an external wound, disease, or aging, has received significant attention over the past decade. These interbody devices are typically synthetic vertebral body spacers that are inserted into the disc space after extraction of the affected intervertebral disc. However, these synthetic devices often fail to reproduce all of the mechanical properties inherent to the intervertebral disc, such as flexibility and load-buffering. In addition, the implant itself has the possibility of damaging the neighboring vertebrae and nerves. This is particularly true for interbody devices that may be dislodged from the disc under load conditions. Hence, it is quite important to prevent movement of the synthetic device after insertion into the patient.
Typically the synthetic interbody device is inserted by first making an opening in the anulus fibrosus and then inserting the interbody device through the opening. While, reduction in size of the opening and the insertion route, there is still an undesirably high chance for movement of the interbody device post implantation. In addition, many of the interbody devices on the market today have at least two bearing surfaces for engaging the two adjacent end plates, where the bearing surfaces often have engagement mechanisms that comprise studs, keels, and/or a porous surface.
A large number of interbody devices have been designed and produced, ranging from large cylindrical dowels to small rectangular wedges. Many of the interbody devices currently in use also have open passages through them (hollow spaces). Examples of interbody devices include, but are not limited to, the Adaptive Vertebral Spacer (AVS) AS, AVS TL, AVS AL, AVS PL, Vertebral Body Support System, and VLIFT Vertebral Body Replacement System, which are sold by Stryker, the DEVEX, LT & ST Mesh systems, and the Leopard, Jaguar and Saber cages, which are sold by DePuy, the BAK Vista interbody fusion system, which is sold by Zimmer Spine, and the INTER FIX™ and INTER FIX™ RP threaded fusion devices and the LT-CAGE® lumbar tapered fusion device, which are sold by Medtronic Sofamor Danek. See also, U.S. Pat. No. 6,923,810, and U.S. Pat. No. 6,758,849.
Because these interbody devices may be extruded, either partially or completely, from the site of implantation, there is a need in the market for a method of holding the device in place and/or filling void spaces between the device and the intended contact surfaces in the patient.
A particular class of polymers that have proven useful in biological applications are hydrogels, which consist of a three-dimensional network of hydrophilic polymers with water filling the space between the polymer chains. Hydrogels may be obtained by copolymerizing suitable hydrophilic monomers, by chain extension, and by cross-linking hydrophilic pre-polymers or polymers.
A thermoreversible hydrogel matrix, which is liquid near physiologic temperatures, has been shown to elicit vasculogenesis and modulate wound healing. This bioactive hydrogel material has also been shown to improve healing in response to implanted foreign materials; demonstrating a decrease in the surrounding fibrous capsule thickness and a persistent increase in blood supply immediately adjacent to implanted materials exposed to this thermoreversible hydrogel. However, that thermoreversible hydrogel is molten at physiologic temperatures, rendering it inappropriate for use in vivo (see WO 2003/072155).
U.S. Pat. No. 5,972,385 describes a matrix comprising a modified polysaccharide with collagen for tissue repair, which can be combined with growth factors.
U.S. Pat. No. 6,287,588 describes a matrix comprising a continuous biocompatible gel phase, such as a hydrogel, and a discontinuous particulate phase, such as microspheres, and a therapeutic agent contained in both phases. The '588 patent does not discuss any use in conjunction with an interbody device.
Additional publications and patents have described polymers and co-polymers for use in medical applications, such as drug delivery, tissue regeneration, wound healing, wound dressings, adhesion barriers, and wound adhesives. For example, see U.S. Pat. No. 4,618,490 and U.S. Pat. No. 6,165,488.